Source : Webster's Revised Unabridged Dictionary (1913)

Heliostat \He"li*o*stat\, n. [Helio- + Gr. ? placed, standing,
   fr. ? to place, stand: cf. F. h['e]liostate.]
   An instrument consisting of a mirror moved by clockwork, by
   which a sunbeam is made apparently stationary, by being
   steadily directed to one spot during the whole of its diurnal
   period; also, a geodetic heliotrope.